1 Answer
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Think of the mechanics of a violin, the way the strings are supported by the curved top of the bridge. Mostly, the player wants to bow just one string without touching the others, the curvature enables this. He can bow two strings at once if he wishes. But not three.

This notation asks for the common gesture where the lower notes are played briefly and left to ring, the upper note is then played normally.

(For the sake of completeness, I should mention that fully bowed triple stops CAN be achieved in certain circumstances.)